The present invention relates to predicting the amount of an aspirated liquid, particularly an aspirated sample used in a diagnostic analyzer.
Known diagnostic analyzers include immunodiagnostic and clinical chemistry analyzers such as the VITROS® ECi immunodiagnostic analyzer, or a clinical chemistry analyzer such as the VITROS® 5,1 FS, both sold by Ortho-Clinical Diagnostics, Inc. All such analyzers are collectively called diagnostic analyzers. Such systems rely on a constant sample volume being delivered to the reaction to give a precise reported assay result. The precision of the sample aspiration is typically known and is often a significant contributor to the precision of reported results. These diagnostic analyzers commonly use pressure detection systems to monitor the aspiration of sample and reagent liquids. The recorded pressure profile is monitored and evaluated in an attempt to determine if foam, bubbles, clots or any other anomaly is observed which could adversely affect the intended aspirate or dispense volume. When a problem is detected, the instrument will alert the operator, and suppress the result. While detecting errors and discarding the results will improve assay precision, it will reduce usable results, resulting in the use of more sample to re-run the analysis.
In an attempt to conserve sample volume, especially in pediatric and geriatric settings, and in an attempt to minimize reagent usage for cost consideration, test volumes are under constant pressure to be reduced. As volumes are reduced below 5 uL, liquid handling system requirements for precision and accuracy are becoming more stringent. Small deviations in delivered volume of liquid have a direct affect on the reaction and result. Some patents describe the art of detecting liquid handling anomalies such as bubbles, clots and foam as well as predicting that an aspirated volume is insufficient to allow reporting of a result. See, e.g., U.S. Pat. No. 6,060,320. U.S. Pat. No. 6,112,605 discloses aspirating microvolumes of a transfer liquid. An air gap is located between the transfer liquid and the system liquid. After a dispense of transfer liquid, the transfer liquid will return to its prior position due to capillary forces. This causes the air gap volume to increase along with a corresponding decrease in pressure. Based on the decrease in pressure of the air gap, the volume of liquid dispensed can be determined. U.S. Pat. Nos. 6,422,431, 6,083,762, 6,220,075, 6,094,966, 5,927,547, 6,079,283 and 6,203,759 all disclose aspirating and/or dispensing liquids using an aspirate/dispense probe. However, none of the known art teaches correcting for small volume errors in aspiration caused by conditions such as viscosity.
For the foregoing reasons, there is a need for a method of aspirating a liquid that can correct for variations in the volume of an aspirated liquid, and hence, more accurately predict the actual volume of aspirated liquid.